1. Field of the Invention
The invention relates to a hydraulic pressure control circuit, and more particularly, to a failure determining apparatus and method for a hydraulic pressure control circuit relating to a lock-up clutch.
2. Description of the Related Art
An automatic transmission mounted in a vehicle typically combines a torque converter and a gear type shift mechanism and is structured so as to automatically establish a predetermined shift speed by switching the power transmission path in the gear type shift mechanism by selectively operating a plurality of friction apply elements such as clutches and brakes.
This automatic transmission is provided with a hydraulic pressure control circuit which controls the supply and release of hydraulic fluid to and from actuators of the friction apply elements and a lock-up clutch of the torque converter.
Japanese Patent Application Publication No. JP-A-2005-3193 describes a torque converter in which, when controlling a lock-up clutch that enables the input side of the lock-up clutch to be directly connected to the output side of the lock-up clutch, in addition to simply controlling the lock-up clutch to an applied state and a released state, it can also be placed in a slip state by feedback controlling the application force of the lock-up clutch so that the rotation speed difference between a pump rotation speed on the input side (which corresponds to the engine speed) and a turbine rotation speed on the output side (which corresponds to an input shaft rotation speed of the automatic transmission) becomes a predetermined rotation speed difference.
In order to control the lock-up clutch to a slip state in this way (i.e., in order to execute slip control), a solenoid valve is provided in a hydraulic pressure control circuit of the lock-up clutch in order to adjust (regulate) the pressure difference between an apply side fluid chamber and a release side fluid chamber of the lock-up clutch. However, if there is an ON failure in the solenoid valve, the lock-up clutch becomes applied despite the fact that it desirable to have it in a slip state and engine stall ensues. Therefore, it is necessary to be able to determine an ON failure in the solenoid valve.
Japanese Patent Application Publication No. JP-A-2004-340273 describes technology for determining an ON failure in a solenoid valve. The technology described therein is an apparatus provided with a hydraulic pressure switch in a hydraulic pressure control circuit, which operates in response to the amount of hydraulic pressure output from the solenoid valve. When the hydraulic pressure switch is on even though no hydraulic pressure should be supplied, the apparatus determines that an ON failure has occurred in the solenoid valve.
However, when using a hydraulic pressure switch to determine an ON failure (i.e., the solenoid valve does not turn off even when an OFF command is issued) in a solenoid valve of a lock-up clutch, it is not possible to identify whether the failure is in the solenoid valve or in the hydraulic pressure switch.
Japanese Patent Application Publication No. JP-A-2004-340273 attempts to solve this problem by identifying whether there is an ON failure in the solenoid valve or an ON failure is in the hydraulic pressure switch based on the operating state of the hydraulic pressure switch when the ignition switch is off and no hydraulic pressure is output from the hydraulic pressure pump. With this technology, however, it is not possible to identify where the failure is until the ignition switch has been turned off, which means that there is a time delay until a failsafe routine targeted at the cause of the failure can be executed.